This invention relates to a fire barrier composition, a substrate coated with the composition, a laminate based on the coated substrate and a method of making the composition.
Numerous methods have been developed for controlling the ravages of fire. The rationale for each of these methods is found in the physics of combustion, the chemistry of flame, and the engineering of fire control systems. The disclosure herein presented treats with the use of coatings as a fire control method; a barrier system intended to prevent the ignition of and spread of flame along a combustible substrate.
The prior art discloses four major types of fire-protective coatings: (1) ablative; (2) intumescent; (3) sublimation and gas- or vapor-producing; (4) "ceramic" or inorganic. Ablative coatings are generally thick and heavy and costly, thus limiting their application to special uses. Intumescent coatings are generally soft and easily abraded, and have limited duration of protection--10 to 15 minutes when exposed to fire. Sublimative or gas-producing coatings have even shorter duration of protection--after their snuffing gases have been produced and wafted away from the surface, they are left without a protective mechanism. They have found their major use in the treatment of fibers and clothing. Ceramic coatings, as the name implies, require high-temperature curing in order to form the ceramic bond,--which many structural or building components cannot withstand. Inorganic coatings, such as Portland cement, gypsum, calcium aluminate cement, phosphate-bonded cement, metal-pigment loaded silicate coatings (sodium, potassium, ethyl, etc.), high-temperature silicone, and magnesium "oxychloride" and magnesium "oxysulphate" cements have all been proposed. This disclosure treats specifically with magnesium "oxychloride" in combination with calcium aluminate cements and other high-temperature resistant binders.
It is remarkable that in the long history of oxysalt cements, so few attempts have been made to convert these materials into a fire barrier in the form of thin paints. No doubt this is, in part, due to the prior major shortcomings of the products and the inability of the developers to overcome them. Amongst these shortcomings are: brittleness and rigidity, along with the proneness to spalling or decrepitating under heat. Unless mixed in stoichiometric ratios, the result is unsightly efflorescence ("white bloom") of the gauging salts (magnesium chloride, magnesium sulphate, magnesium carbonate) which continually condenses onto the surface of the coating. In other cases, especially in the oxysulphates, the excess liquids tend to synerese (squeeze out) and leave a greasy wet film at the surface. The coatings exhibit poor aging and weatherability characteristics, as well as solubility in water and rain, thereby making them unusable in outdoor situations. (The rain and high humidity leach the chloride and sulphate from the formulation, leaving void spaces as the result of the leaching process, which in turn leads to a disintegration of the coating.) They have a strong tendency to be hygroscopic, picking up moisture from the air, and thereby presenting a continuously wetted surface; they show poor resistance to common solvents; and there is a loss of tensile strength and cohesiveness after exposure to flame, causing cracking which exposes the substrate to ignition and subsequent combustion. In addition, some coatings are relatively soft and easily scratched and abraded, and unable to resist scrubbing (oxysulphate coatings are markedly softer than oxychloride coatings). Thin coatings, although they adhere well to substrates, frequently shrink in drying, and show "mud-cracking" (crazing), thus exposing the underlying substrate to direct flame impingement.
Thus, there exists a need in the art to develop a durable, quick-setting, non-combustible thin coating capable of protecting a variety of susceptible flammable substrates from ignition ("fire-barrier"), one which at the same time has all the attributes of a good paint and, additionally, aids in overcoming the above-listed shortcomings in the prior art. Another need is to develop a "fire-barrier" coating which does not spall or decrepitate or crack under the heat of flame and thus expose the underlying substrate. A further need is to develop an inexpensive Class "A" fire-retardant coating as defined by the requirements of the National Fire Protection Association (NFPA), utilizing inorganic non-toxic components, and based on magnesium oxide-magnesium chloride "oxysalt" chemistry. Other needs are to develop a coating which would bond to wood, plywood, and gypsum (plaster wallboard, to fiberglass board and batting, and to other substrates, imparting to them fire-barrier properties and increased flexural strength when incorporated as an element of the laminate structures. In addition, a barrier coating must possess a suite of other physical characteristics: hardness, compressive and tensile strengths, cohesiveness, adhesion, and a variety of other properties, including cosmetic attributes, which keeps the coating's integrity under the stresses of fire exposure, so that it does not crack, spall, flake off, or otherwise lose its ability to act as a fire barrier. The coating itself must be non-combustible, so that it does not contribute to the fuel, nor aid the progress of a flame front.